Project Summanry/Abstract Although abiraterone (Zytiga) and enzalutamide (Xtandi) represent significant advances in the treatment of metastatic castration-resistant prostate cancer (mCRPC), all eventually acquire drug resistance over time and become lethal. Therefore, new research strategy and intervention are urgently needed to develop effective therapy for CRPC. We have shown that galectin-3 (Gal3), a beta-galactoside-binding lectin, is involved in tumor progression and metastasis in prostate cancer. To target Gal3, we developed a very potent (picomolar affinity) and specific Gal3 antagonist, TFD100, which not only blocks metastasis, but also promotes anti-tumor immune response in our transgenic mouse model of metastatic prostate cancer (Hoxb13/MYC/Ptenlox/lox BMPC transgenic mice). Moreover, Gal3 contributes to drug resistance. Unlike PC-3 and DU-145, LNCaP prostate cancer cells do not constitutively express Gal3. To explore the mechanisms of drug resistance, we created abiraterone resistant LNCaP (Abi-LNCaP) and enzalutamide resistant LNCaP (Enza-LNCaP) cells. Interestingly, Abi-LNCaP and Enza-LNCaP cells inducively express Gal3 and become sensitive to Abi and Enza in the presence of TFD100. Based on the compelling preliminary data we hypothesize that specific (on- target) inhibition of Gal3 with TFD100 will suppress not only tumor growth and metastasis, but also overcome drug resistance by impeding Gal3-mediated signaling and by promoting anti-tumor immune response. To test this hypothesis, we will first produce recombinant TFD100 from engineered CHO cells. We will then define the mechanism(s) of Gal3-mediated drug resistance and sensitize drug resistant cells with TFD100. This includes epigenetic modification of Gal3 promoter and various signaling mechanisms such as androgen receptor (AR), AR variant, P-glycoprotein, PD-L1. To establish effective dose range of TFD100 for in vivo experiments, we will determine TFD100?s maximum tolerated dose (MTD) in NSG mice. We will then examine TFD100?s ability to prevent tumor growth in a patient mCRPC tumor tissue derived xenograft (PDX) in humanized NSG mice (hu- NSG) model alone and in conjunction with Abi. After drug treatment, tumor growth will be measured. Histochemical analyses of the tumors will be performed with various gene markers including Gal3 and AR. This study, for the first time, will explore the therapeutic utility of a natural carbohydrate compound to overcome drug resistance in mCRPC therapy. TFD100 is anticipated to be a significant advancement in the arsenal against mCRPC, exerting a ?multi-pronged? attack on tumors, which is expected to result in longer survival of mCRPC patients with improved quality of life.